Large static first and second hyperpolarizabilities dominated by excess electron transition for radical ion pair salts M₂ ^•+TCNQ^•- (M = Li, Na, K)

The interesting radical ion pair salts M₂ ^•+TCNQ^•- (M = Li, Na, K) are a particular class of charge transfer complexes with excess electron. The ground states of these complexes are triplet. The C_2v symmetry geometrical structures of the M₂^•+TCNQ^•- (M = Li, Na, K) with all-real frequencies are obtained at the density functional theory (DFT) B3LYP/6-31+G(d) level. All calculations of electric properties in this paper have been carried out at the restricted open-shell second order Møller-Plesset perturbation theory (ROMP2) level. Owing to existing excess electron (from the polarized alkali metal atoms) these charge transfer complexes exhibit large nonlinear optical (NLO) responses dominated by excess electron transitions. For these radical ion pair salts M₂ ^•+TCNQ^•-, the static first hyperpolarizabilities (β₀) are large. The order of β₀ values is 19 203 (M = Li) < 24 140 (M = Na) < 29 065 a.u. (M = K). Specially, the second hyperpolarizability (γ₀) of the complexes with excess electron is obtained for the first time. These static second hyperpolarizabilities are also large. The order of γ₀ values is 2 213 006 (M = Li) < 3 136 754 (M = Na) < 7 905 623 a.u. (M = K). Among the three structures, K₂^•+TCNQ^•- has the largest γ₀ value to be 7.9 × 10⁶ a.u. (3982 × 10^-36 esu), which is about 9 times larger than that of the intramolecular charge transfer complex σ-arylvinylidene trans-[Ru(4-C=CHC₆H₄C=CC₆H₄NO ₂)Cl(dppm)₂]PF₆ [Hurst et al., Organometallics, 2001, 20, 4664]. The present investigation provides a new kind of candidates for the high-performance NLO materials.